materials Article Faraday Rotation of Dy2O3, CeF3 and Y3Fe5O12 at the Mid-Infrared Wavelengths David Vojna 1,2,* , OndˇrejSlezák 1 , Ryo Yasuhara 3 , Hiroaki Furuse 4 , Antonio Lucianetti 1 and Tomáš Mocek 1 1 HiLASE Centre, FZU–Institute of Physics of the Czech Academy of Sciences, Za Radnicí 828, 252 41 Dolní Bˇrežany, Czech Republic; [email protected] (O.S.); [email protected] (A.L.); [email protected] (T.M.) 2 Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Bˇrehová7, 115 19 Prague, Czech Republic 3 National Institute for Fusion Science, National Institutes of Natural Sciences, 322-6, Oroshi-cho, Toki, Gifu 509-5292, Japan; [email protected] 4 Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan; [email protected] * Correspondence: [email protected] Received: 5 October 2020; Accepted: 23 November 2020; Published: 24 November 2020 Abstract: The relatively narrow choice of magneto-active materials that could be used to construct Faraday devices (such as rotators or isolators) for the mid-infrared wavelengths arguably represents a pressing issue that is currently limiting the development of the mid-infrared lasers. Furthermore, the knowledge of the magneto-optical properties of the yet-reported mid-infrared magneto-active materials is usually restricted to a single wavelength only. To address this issue, we have dedicated this work to a comprehensive investigation of the magneto-optical properties of both the emerging (Dy2O3 ceramics and CeF3 crystal) and established (Y3Fe5O12 crystal) mid-infrared magneto-active materials. A broadband radiation source was used in a combination with an advanced polarization-stepping method, enabling an in-depth analysis of the wavelength dependence of the investigated materials’ Faraday rotation. We were able to derive approximate models for the examined dependence, which, as we believe, may be conveniently used for designing the needed mid-infrared Faraday devices for lasers with the emission wavelengths in the 2-µm spectral region. In the case of Y3Fe5O12 crystal, the derived model may be used as a rough approximation of the material’s saturated Faraday rotation even beyond the 2-µm wavelengths. Keywords: Faraday effect; magneto-optical properties; Verdet constant; Faraday rotation; magneto-active materials; yttrium iron garnet; dysprosium sesquioxide; cerium fluoride; Faraday devices; mid-infrared lasers 1. Introduction A considerable amount of attention from the scientific community is currently directed towards the development of the mid-infrared (mid-IR) lasers. The main reason is that these laser sources possess the key properties for a vast number of technological applications (e.g., LiDAR, gas sensing, material processing, or pump sources of optical parametric oscillators [1–3]) and are enabling further advancement of the basic research as well (e.g., in the attosecond science for efficient high harmonic X-ray generation [4–6]). The huge demand for the mid-IR lasers has inevitably led to an intensified development of the corresponding laser sources and optical components [1,7–12]. One of the most commonly used components in laser systems are those based on the Faraday effect, incorporating, for instance, magneto-optical modulators, polarization switches, rotators, deflectors, or isolators. All of these are frequently used for the polarization-based signal processing Materials 2020, 13, 5324; doi:10.3390/ma13235324 www.mdpi.com/journal/materials Materials 2020, 13, 5324 2 of 17 of the laser beam, construction of multi-pass amplifiers, or as a protection of the laser system from back-reflections [13–15]. The fundamental part of these devices is a piece of magneto-active material in which the plane of the polarized radiation is rotated by a certain angle as a result of applying the magnetic field. A great portion of research dedicated to the development of Faraday devices now focuses on the investigation of new magneto-active materials, which would enable running these devices with high efficiency. The basic requirements on an efficient magneto-active material are to exhibit a high Faraday rotation, which is represented by the Verdet constant or the specific Faraday rotation parameters, and low absorption at the incident laser wavelength. In the case of a Faraday device designed for a high average power laser, the list of the material parameters which needs to be further assessed incorporates the thermo-optical, thermo-mechanical and elasto-optical properties of the material [16–19]. While a lot of scientific effort has been already put into the investigation of the magneto-active materials for the visible and near-IR wavelengths, the options for the mid-IR wavelengths are still very limited [18]. The materials used for the near-IR wavelengths usually include Tb3+ ions (e.g., the terbium-based garnets, fluorides or sesquioxides have been reported [20–23]), which have strong absorption in the mid-IR and, therefore, cannot be used. For low-power mid-IR laser applications, the ferrimagnetic yttrium iron garnet Y3Fe5O12 (YIG) crystal and its rare-earth-substituted compositions may be used, as these possess a very high specific Faraday rotation and low saturation magnetization [13,24]. Nevertheless, the fabrication of these crystals is a relatively time-consuming process due to the need for using the floating zone or liquid phase epitaxial methods. These drawbacks have recently prompted researchers to explore the possibility of fabricating the iron garnets in the form of transparent ceramics which would enable reducing the time needed for obtaining the iron-garnet-based magneto-optical elements as well as to manufacture them in larger sizes [25–27]. Most recently, quite a few paramagnetic or diamagnetic mid-IR magneto-active materials have been reported as well. Unlike to the ferrimagnetic iron garnets, the saturation of the Faraday effect is much less pronounced in these materials and, hence, they may be used in the linear regime of operation in which the polarization plane rotation angle is tuned by adjusting the applied magnetic field rather than by adjusting the length of the magneto-optical element. Concerning the paramagnetic materials, favourable optical characteristics for the mid-IR region were reported for fluorides including rare-earth ions [28], specifically for the CeF3, PrF3 and LiREF4 (RE = Dy, Ho or Er). Among these, to the best of our knowledge, only the properties of the CeF3 crystal were further investigated in the context of the Faraday devices [29–32]; and, concerning the mid-IR, its Verdet constant is still known only up to 1.95 µm [31]. Very recently, the mid-IR magneto-optical characteristics were reported for the (DyXY0.95−XLa0.05)2O3 ceramics [33] (with variable doping X = 0.7, 0.8 and 0.9), EuF2 crystal [34], Dy-doped multicomponent glasses [35] and for a tellurium-arsenic-selenium glass [36], all of them showing a great promise for the mid-IR region. In these papers, the room temperature Verdet constant on a single wavelength of ∼1.94 µm was reported for all of the mentioned materials to be in the range of ∼8–15 rad/Tm in the absolute value (see Table1). After the needed optimization of the fabrication process and further characterization, these materials may be used for the mid-IR Faraday devices, nevertheless, given the reported values of the Verdet constant, their use might require relatively strong magnetic fields >2 T. Permanent magnets with such peak magnetic field magnitudes were already reported [37–39]; however, it might be quite challenging to achieve the desired axial magnetic field over the whole needed length of the magneto-optical element. Therefore, searching for a non-ferrimagnetic mid-IR magneto-active material with higher magneto-optical characteristics still represents an up-to-date task. Materials 2020, 13, 5324 3 of 17 Table 1. The yet-reported paramagnetic or diamagnetic mid-IR magneto-active materials and the absolute values of their Verdet constant at ∼1.94 µm [31,33–36]. Material jVj [rad/T.m] @ ∼1.94 µm Magnetic Behaviour CeF3 crystal 10.1 paramagnetic (DyXY0.95−XLa0.05)2O3 ceramics (X = 0.7–0.9) 10.7–13.8 paramagnetic EuF2 crystal 12.3 paramagnetic 75 % wt. Dy3+-doped multicomponent glass 7.9 paramagnetic Te20As30Se50 glass 15.2 diamagnetic In our recent work [40], we have investigated the Verdet constant of a pure Dy2O3 ceramics as a function of wavelength and temperature. Compared to the Dy-Y-La-based ceramics [33], the pure 3+ Dy2O3 has a larger concentration of the highly magnetically active Dy ions, and, thereby, a larger magneto-optical activity was observed at the visible and near-IR wavelengths. In the study, we have specifically demonstrated the importance of consideration of the Dy3+ ion’s multiple transitions contribution to the Verdet constant of the Dy2O3 ceramics, nevertheless, the obtained data in the mid-IR suffered from a large measurement error due to the limited optical quality and short length of the characterized sample. In this paper, we report on the fabrication of a new material sample of Dy2O3 ceramics and a comprehensive investigation of the magneto-optical properties with a focus on the mid-IR region. Apart from the Dy2O3 ceramics, we have also investigated samples of YIG and CeF3 crystals under the same experimental conditions. The knowledge of the magneto-optical properties of both of these materials is incomplete in their mid-IR transparency windows, which, although these materials are commercially available, is currently hampering their practical implementation in the mid-IR Faraday devices. For all samples under investigation, we have measured the optical in-line transmittance (0.3–3 µm) and the Verdet constant dispersion (0.6–2.3 µm for the Dy2O3 ceramics and CeF3 crystal) or the specific Faraday rotation (1.1–2.3 µm for the YIG crystal) at the room temperature.